First, you must be subscribed to this mod for it to work; you will be modifiying this mod to make it do so.

Step 1:
In File Explorer, go to the hard drive XCOM 2 is installed on and navigate through the following files: SteamLibrary\SteamApps\workshop\content\268500\642086911\Config

Step 2:
Open the file called XComGameBoard.ini. If windows asks which program to load the file with, select notepad.

Step 3:
In the last line of the first paragraph, change UnitWeight=0 to something rediculously high. The way I figured this UnitWeight system works, is if UnitWeight=1, then there's a 1 to 1 chance a newly generated unit will be from that country. So setting it to anything over 1000 (making the odds of the unit not being from this country being 1000 to 1) should make the game create every unit from this country.

Step 4:
Save and Close. Congrats, you have modded a mod.

Note 1:
Not only will this force all newly generated recruit and character pool unit to be a Rookie, but it will force all soldiers rewarded to you in the game to be a Rookie as well.

Note 2:
Any units in the recruit or character pool will not be changed to be from the Rookie nation. They will stay from their original country. This change will only affect newly generated units.

Note3:
This does not prevent units being generated from other nations, but makes it so unlikely that it seems impossible.